1. Field of the Invention
The present invention relates to an optical transmission line employed in a repeating transmission line disposed between stations, and an optical transmission system including the same.
2. Related Background Art
Wavelength division multiplexing (WDM) optical transmission utilizing signals of a plurality of channels included in a 1.55-xcexcm wavelength band enables high-speed, large-capacity information transmissions. Factors restricting the transmission capacity in this WDM optical transmission include the nonlinearity and dispersion slope of the optical transmission line. Therefore, in order to improve the performance of a WDM optical transmission system, it is important to suppress the nonlinearity of the optical transmission line (e.g., by increasing its effective area) and lower the dispersion slope of the optical transmission line.
Proposed as an optical transmission line aimed at suppressing the nonlinearity and lowering the dispersion slope as such is an optical transmission line having a configuration in which a single-mode optical fiber and a dispersion-compensating optical fiber are connected to each other. The single-mode optical fiber (hereinafter referred to as SMF) has a zero-dispersion wavelength in a 1.3-xcexcm wavelength band and exhibits, in the 1.55-xcexcm wavelength band, a positive chromatic dispersion and a positive dispersion slope. On the other hand, the dispersion-compensating optical fiber (hereinafter referred to as DCF) exhibits, in the 1.55-xcexcm wavelength band, a negative chromatic dispersion and a negative dispersion slope. Hence, the respective lengths of the SMF and DCF are appropriately adjusted, so as to lower the dispersion slope of the optical transmission line as a whole. Also, since the SMF having a relatively large effective area is disposed on the upstream side in the signal propagating direction, the effective area of the whole transmission line is enhanced, and the nonlinearity of the optical transmission line is suppressed.
For example, the conventional optical transmission line disclosed in T. Naito, et. al, xe2x80x9c1 Terabit/s WDM Transmission over 10,000 km,xe2x80x9d ECOC""99, PD-2-1 (1999), hereinafter referred to as first conventional technique, comprises a configuration in which an SMF and a DCF are connected to each other. The conventional optical transmission line disclosed in Chikutani, et al., xe2x80x9cLow Nonlinear PSCF+DCF Complex Transmission Line having Low Dispersion Slope and Low Nonlinearity,xe2x80x9d IEICE Technical Report, OCS99-97, pp. 67-72 (1999), hereinafter referred to as second conventional example, comprises a configuration in which an SMF (hereinafter referred to as Aeff-enlarged PSCF) exhibiting an effective area Aeff greater than a commonly known value thereof and having a core region made of pure silica (non-intentionally doped silica), and a DCF are connected to each other. The conventional optical transmission line disclosed in M. Murakami, et al., xe2x80x9cQuarter Terabit (25xc3x9710 Gb/s) over 9288 km WDM Transmission Experiment Using Nonlinear Supported RZ Pulse in Higher Order Fiber Dispersion Managed Line,xe2x80x9d ECOC""98, PD, pp. 79-81 (1998), hereinafter referred to as third conventional example, comprises a configuration in which an SMF (hereinafter referred to as Ge-SM) having a core region doped with Ge and a DCF are connected to each other.
The conventional optical transmission line disclosed in K. Fukuchi, et al., xe2x80x9c1.1-Tb/s (55xc3x9720-Gb/s) Dense WDM Soliton Transmission Over 3,020-km Widely-Dispersion-Managed Transmission Line Employing 1.55/1.58-xcexcm Hybrid Repeaters,xe2x80x9d ECOC""99, PD-2-10 (1999), hereinafter referred to as fourth conventional example, comprises a configuration in which an SMF (hereinafter referred to as PSCF (Pure Silica Core Fiber)) having a core region made of pure silica and a DCF are connected to each other. The conventional optical transmission line disclosed in T. Tsuritani, et al., xe2x80x9c1 Tbit/s (100xc3x9710.7 Gbit/s) Transoceanic Transmission Using 30 nm-Wide Broadband Optical Repeaters with Aeff-Enlarged Positive Dispersion Fibre and Slope-Compensating DCF,xe2x80x9d ECOC""99, PD-2-7 (1999), hereinafter referred to as fifth conventional example, comprises a configuration in which an Aeff-Enlarged PSCF and a DCF are connected to each other.
The inventors studied the above-mentioned optical transmission lines according to the first to fifth conventional examples and, as a result, have found the following problems. Namely, effects of fully lowering the nonlinearity and dispersion slope may not be obtained in the optical transmission lines according to the first and second conventional examples since their bending loss is about 1 dB/m so that they are designed to become excessively resistant to bending. In the optical transmission lines according to the third and fourth conventional examples, the effect of lowering the nonlinearity may not: fully be obtained since the relative refractive index difference of the core region in the DCF is assumed to be about 1.2%. The effect of fully lowering the nonlinearity may not be expected in the optical transmission line according to the fifth conventional example, since the relative refractive index difference of the core region in the DCF is assumed to be about 2.0%. Here, none of the optical transmission lines according to the third to fifth conventional examples is optimized in terms of the ratio of length of DCF in the whole optical transmission line, and the like.
In order to overcome the problems mentioned above, it is an object of the present invention to provide an optical transmission line comprising a structure for effectively lowering both the nonlinearity and dispersion slope, and an optical transmission system including the same.
The optical transmission line according to the present invention is a repeating transmission line which has a predetermined span length and is disposed between stations, such as transmitting stations, repeater stations, and receiving stations, as a transmission medium suitable for WDM optical transmission utilizing signals of a plurality of channels different from each other. This optical transmission line comprises a single-mode optical fiber having a zero-dispersion wavelength in a 1.3-xcexcm wavelength band, and a dispersion-compensating optical fiber for compensating for a chromatic dispersion of the single-mode optical fiber. The single-mode optical fiber and the dispersion-compensating optical fiber are successively disposed in this order along a signal propagating direction and are fusion-spliced to each other. The optical transmission line as a whole has an average dispersion slope Save of xe2x88x920.0113 ps/nm2/km or more but 0.0256 ps/nm2/km or less at a wavelength of 1550 nm, and an equivalent effective area EAeff of 50 xcexcm2 or more at the wavelength of 1550 nm.
In particular, the above-mentioned average dispersion slope Save and equivalent effective area EAeff in the optical transmission line according to the present invention satisfy the following relationship:
f(Save)xe2x89xa6EAeffxe2x89xa6g(Save)xe2x80x83xe2x80x83(1) 
where f(Save) is a lower limit function which yields the lower limit of EAeff by the expression:                               0.4481          +                                                                                        (                0.4481                )                            2                        -                          4              xc3x97              0.00518              ⁢                              xe2x80x83                            xc3x97                              [                                  3.29                  -                                      ln                    ⁡                                          (                                                                        S                          ave                                                +                        0.0053                                            )                                                                      ]                                                              2    xc3x97    0.00518  
while using Save as a variable, and g(Save) is an upper limit function which yields the upper limit of EAeff by the expression:             0.4481      +                                                                                                        (                    0.4481                    )                                    2                                -                                  4                  xc3x97                  0.00518                  ⁢                                      xe2x80x83                                    xc3x97                                      {                                          3.29                      -                      ln                                                                                                                                                                [                                                            S                      ave                                        +                    0.0053                    +                                          0.016                      ⁢                                              (                                                                              log                            ⁢                                                          xe2x80x83                                                        ⁢                            10                                                    -                                                      log                            ⁢                                                          xe2x80x83                                                        ⁢                            2                                                                          )                                                                              ]                                }                                                                2      xc3x97      0.00518        +      12    ⁢          (                        log          ⁢                      xe2x80x83                    ⁢          10                -                  log          ⁢                      xe2x80x83                    ⁢          2                    )      
while using Save as a variable.
The relationship represented by the above-mentioned expression (1) indicates an appropriate range of equivalent effective area EAeff for controlling the bending loss within the range from 2 dB to 10 dB as a permissible range at a span length of 50 km in order to enable high-speed, large-capacity WDM optical transmission not only in C band (having a wavelength of 1530 to 1565 nm) but also in L band (having a wavelength of 1565 to 1625 nm)
Thus, this optical transmission line is a repeating transmission line in which a single-mode optical fiber and a dispersion-compensating optical fiber are fusion-spliced to each other, in which signals successively propagate through the single-mode optical fiber and dispersion-compensating optical fiber in this order. At the wavelength of 1550 nm, the single-mode optical fiber and dispersion-compensating optical fiber have respective chromatic dispersions with polarities different: from each other and respective dispersion slopes with polarities different from each other, whereby the absolute value of chromatic dispersion and the absolute value of dispersion slope become smaller in the optical transmission line as a whole. When the average dispersion slope Save and equivalent effective area EAeff in the whole optical transmission line are set to satisfy the above-mentioned range, both the nonlinearity and average dispersion slope of the optical transmission line are lowered effectively, whereby a high bit rate (e.g., about 10 Gbits/s) of WDM transmission (high-speed, large-capacity optical transmission) is possible over a wider wavelength band, e.g., from 1530 nm to 1600 nm.
Preferably, the average dispersion slope Save is 0.0021 ps/nm2/km or less. In this case, high-speed, large-capacity WDM transmission is possible at a higher bit rate (e.g., about 20 Gbits/s). The equivalent effective area EAeff is preferably 55 xcexcm2 or more (more preferably 60 xcexcm2 or more), whereby the nonlinearity of the optical transmission line is further lowered.
In addition, it is preferred that the optical transmission line as a whole have an average transmission loss of 0.185 dB/km or more but 0.210 dB/km or less at the wavelength of 1550 nm. Preferably, in the wavelength band from 1530 nm to 1600 nm, the average transmission loss is 0.185 dB/km or more but 0.220 dB/km or less. In each case, the transmission loss of the optical transmission line is sufficiently small, so that the input signal power can be made lower, whereby signal waveforms can effectively be restrained from deteriorating due to nonlinear effects.
In the single-mode optical fiber, the effective area Aeff at the wavelength of 1550 nm is preferably 100 xcexcm2 or more. While the signal power density decreases as the effective area increases, signal waveforms are restrained from deteriorating due to nonlinear effects, whereby the equivalent effective area EAeff becomes greater. Preferably, the single-mode optical fiber has a core region made of pure silica not doped with GeO2. This is because of the fact that, since the transmission loss caused by Rayleigh scattering is lower in the core region (the transmission loss of the whole optical transmission line is lower), the input signal power can be suppressed, whereby the equivalent effective area EAeff becomes greater.
In the dispersion-compensating optical fiber, its core region preferably has a relative refractive index difference of 1.4% or more but 1.8% or less with respect to a reference region. This is because of the fact that the equivalent effective area EAeff of the optical transmission line becomes greater (95% or more of the maximum equivalent effective area). The ratio of length occupied by the dispersion-compensating optical fiber in the optical transmission line (repeating transmission line) preferably falls within the range from 23% to 36%, whereas the dispersion-compensating optical fiber preferably has a chromatic dispersion of xe2x88x9281 ps/nm/km or more but xe2x88x9236 ps/nm/km or less at the wavelength of 1550 nm. This is because of the fact that the equivalent effective area EAeff Of the optical transmission line can become 95% or more of the maximum equivalent effective area in these cases.
Preferably, the optical transmission line according to the present invention as a whole has a negative average chromatic dispersion at the wavelength of 1550 nm. This is because of the fact that the unstableness in modulation can be suppressed, whereby signal waveforms can effectively be restrained from deteriorating due to cross-phase modulation.
The optical transmission system according to the present invention is suitable for a WDM optical transmission system for enabling large-capacity optical communications and comprises, at least, a receiving station and a transmitting station. One or more repeater stations may be disposed between the receiving station and the transmitting station. The optical transmission line comprising the above-mentioned structure according to the present invention is employed as a repeating transmission line disposed between the above-mentioned stations in at least one of repeating transmission lines between a receiving station and a repeater station, between repeater stations, and between a repeater station and a receiving station. When no repeater station exists between a transmitting station and a receiving station, the optical transmission line is employed in the repeating transmission line between the transmitting station and the receiving station.
Since the absolute value of chromatic dispersion and the absolute value of dispersion slope in the whole optical transmission line are set smaller, and both the nonlinearity and average dispersion slope of the optical transmission line are lowered, a high bit rate (10 Gbits/s) of WDM transmission is possible over a wide wavelength band, e.g., from 1530 nm to 1600 nm.
The optical transmission system according to the present invention may also be configured such that the optical transmission line having the above-mentioned structure (exhibiting a negative chromatic dispersion at the wavelength of 1550 nm) is employed in each of a plurality of repeating transmission lines continuous to each other by way of repeater stations and the like, whereas an optical transmission line constituted by a single-mode optical fiber alone is employed in a repeating transmission line subsequent thereto. In this case, the absolute value of the average chromatic dispersion in the whole optical transmission system can be made smaller, whereby signal waveforms can effectively be restrained from deteriorating due to cumulative chromatic dispersion.
In typical optical transmission systems, an EDFA (Erbium-Doped Fiber Amplifier) is often utilized as an optical amplifier installed in each repeater station. However, it has recently been proposed to elongate the repeating distance by utilizing a Raman amplifier as an optical amplifier.
The optical transmission line according to the present invention, in particular, can suppress the nonlinearity by elongating the span length (repeating distance) between stations, since it comprises a single-mode optical fiber having a zero-dispersion wavelength in the 1.3-xcexcm wavelength band and a dispersion-compensating optical fiber for compensating for the chromatic dispersion of the single-mode optical fiber. Also, the span length, which has been about 50 km in typical submarine cables, can be elongated to 80 km or more by employing a Raman amplifier as an optical amplifier installed in a repeater station.
From the foregoing, it is preferred that the optical transmission line according to the present invention have the above-mentioned equivalent effective area EAeff and a span length L (km) satisfying the relationship of:
0.981xc2x7L+C1xe2x89xa6EAeffxe2x89xa60.981xc2x7L+C2xe2x80x83xe2x80x83(2) 
where C1 is a function of Save given by the expression of f(Save)xe2x88x920.981xc3x9750=f(Save)xe2x88x9249.05, and C2 is a function of Save given by the expression of g(Save)xe2x88x920.981xc3x9750=g(Save)xe2x88x9249.05.
Since the relationship between the equivalent effective area EAeff and the span length L is linearly approximated by the expression of
EAeff=0.98xc3x97L+C(const), 
the above-mentioned expression (2) is derived from this relational expression and the above-mentioned expression (1) in which the span length is supposed to be 50 km.
Therefore, when employed in an optical transmission system in which a Raman amplifier is installed in a repeater station, this optical transmission line has a total length greater than 50 km as a span length. In the case where the optical transmission line is employed in an optical transmission system including a Raman amplifier as such, on the other hand, the dispersion-compensating optical fiber preferably has a core region exhibiting a relative refractive index difference of 1.4xc2x10.2%, more preferably 1.4xc2x10.1%, with respect to a reference region.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.